This invention relates to electromagnetic flowmeters comprising a flow sensor, control electronics, and evaluation electronics. In the following, only flowmeters or flow sensors will be spoken of if necessary for simplicity, and to a method of operating the electromagnetic flowmeter.
As is well known, electromagnetic flowmeters measure the volumetric flow rate of an electrically conductive liquid flowing through a pipe; thus, per definitionem, the liquid volume flowing through a pipe cross section per unit time is measured.
The flowmeter has a, usually nonferromagnetic, flow tube which is connected into the pipe in a liquid-tight manner, e.g., by means of flanges or threaded joints. The portion of the flow tube which contacts the liquid is generally electrically nonconductive, so that a voltage induced in the liquid according to Faraday""s law of electromagnetic induction by a magnetic field cutting across the flow tube will not be short-circuited.
Therefore, metal flow tubes are commonly provided with a nonconductive lining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and are generally nonferromagnetic; in the case of flow tubes made completely of plastic or ceramic, particularly of alumina ceramic, the nonconductive lining is not necessary.
The magnetic field is produced by means of a coil assembly comprising at least two field coils, each of which is positioned on the flow tube along a diameter of the latter. The field coils may be air-core coils or coils with a core of soft magnetic material.
To ensure that the magnetic field produced by the field coils is as homogeneous as possible, the coils are, in the most frequent and simplest case, identical and electrically connected in series, so that in operation they can be traversed by the same excitation current. It is also known to cause the same excitation current to flow through the field coils alternately in the same and the opposite direction in order to be able to determine a flow profile and/or a liquid level in the pipe, see U.S. Pat. No. 5,493,914, or in order to be able to measure the viscosity of non-Newtonian fluids, see U.S. Pat. No. 5,646,353.
The excitation current just mentioned is produced by control electronics; it is regulated at a constant value of, e.g., 85 mA, and its direction is periodically reversed; this serves in particular to largely compensate electrochemical interference voltages developed at the electrodes. The current reversal is achieved by incorporating the field coils in a so-called T network or a so-called H network; for the current regulation and current reversal, see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.
The aforementioned induced voltage is picked off by means of at least two galvanic, i.e., liquid-wetted, electrodes, or by means of at least two capacitive electrodes, i.e., two electrodes disposed in the wall of the flow tube, for example, which in the most frequent case are arranged at diametrically opposed positions such that their common diameter is perpendicular to the direction of the magnetic field and, thus, perpendicular to the diameter on which the field coils are located. The induced voltage is conditioned by means of evaluation electronics to obtain a volumetric flow rate signal, which is recorded, displayed, or further processed.
Electromagnetic flowmeters measure volumetric flow rate with optimum accuracy if the flow in the flow tube is uniformly turbulent. Under this condition of uniform turbulence, each flowmeter is calibrated by the manufacturer, and the values of the so-called calibration factor and the zero drift, which are determined during this calibration, are electronically stored in the flowmeter.
To ensure that after its sale, the flowmeter can be operated with this accuracy in the field, the manufacturer generally specifies an undisturbed inlet section, which is a straight tube length and must be present or be inserted between the flowmeter and a spot of the pipe which disturbs or may disturb the uniform turbulence. Such pipe spots are, for instance, elbows, valves, etc.
During operation of the flowmeter, however, the uniformly turbulent flow profile thus generated may become nonuniform despite the inlet section as a result of unforeseeable events or changes in the liquid. This makes the measurement results inherently more inaccurate and in the worst case even may invalidate the measurement result without this being noticeable.
It is therefore desirable to detect such accuracy-reducing events during measurements, i.e., to derive a corresponding error signal, which then is displayed, triggers an alarm, or serves to correct the measurement result, etc.
To determine the flow profile, but particularly to compensate disturbances in the flow profile, U.S. Pat. No. 5,325,724 proposes to cause excitation currents to flow through both field coils, which produce equidirectional, but temporarily differently strong partial magnetic fields. Investigations have shown, however, that these solutions proposed in U.S. Pat. No. 5,325,724 do not produce the desired effect, namely a significantly asymmetric magnetic field, particularly in the area of the measuring electrodes.
It is therefore an object of the invention to provide a method and a flowmeter for carrying out this method whereby in operation a significantly asymmetric magnetic field can be produced, so that the flow profile can be monitored with high reliability.
To attain this object, the invention provides a method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of:
causing the fluid to flow through the flow tube;
causing a first excitation current of predeterminable strength, generated by means of a measuring and control circuit of the flowmeter, to flow through a first field coil mounted on the flow tube for producing a first partial magnetic field of predeterminable average strength which cuts through the fluid;
causing a second excitation current of predeterminable strength, generated by means of the measuring and control circuit, to flow through a second field coil mounted on the flow tube for producing a second partial magnetic field of predeterminable average strength which also cuts through the fluid;
varying the strength of at least one of the excitation currents in such a manner that the average strengths of the partial magnetic fields are at least temporarily different from each other;
reversing the polarity of one of the two excitation currents in such a manner that the two partial magnetic fields are at least temporarily directed opposite to each other while having different average strengths;
inducing a voltage in the moving fluid traversed by the partial magnetic fields for changing potentials applied to measuring electrodes positioned at the flow tube; and
picking off potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid.
Furthermore, the invention provides a method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and moving fluid, said flowmeter having a flow sensor comprising:
a flow tube for the moving fluid, of which an inner portion, which contacts the fluid, is electrically nonconductive, and which has a tube wall;
a first electrode positioned at or in the flow tube and a second electrode positioned at or in the flow tube, which electrodes are located on a first diameter of the flow tube;
a coil assembly, mounted on the flow tube and comprising a first field coil and a second field coil,
said coil assembly being located on a second diameter of the flow tube, which is perpendicular to the first diameter, and being operable to produce a magnetic field cutting across the tube wall and the fluid when a first excitation current flows in the first field coil and a second excitation current flows in the second field coil,
said excitation currents changing their amplitude and direction periodically during each cycle of the excitation currents such that
during a first quarter cycle, the excitation currents
are equal,
have a constant value, and
flow through the field coils in the same direction, a first direction,
during a second quarter cycle,
the first excitation current has the constant value and
flows through the first field coil in an opposite direction to the first direction,
the second excitation current is less than the constant value by a constant amount and
flows through the second coil in the first direction,
during a third quarter cycle, the excitation currents
have the constant value and
flow through the field coils in the opposite direction, and
during a fourth quarter cycle,
the first excitation current has the constant value and
flows through the first field coil in the first direction, and
the second excitation current is less than the constant value by the constant amount and
flows through the second field coil in the opposite direction,
wherein
first, second, third and fourth voltages are formed from the two potentials during the first, second, and fourth quarter cycles, respectively,
a first voltage difference is formed from the first and third voltages, which serves to compute a volumetric flow rate signal,
a second voltage difference is formed from the second and fourth voltages,
a quotient is formed from the second and first voltage differences,
the quotient is determined during a calibration step of the electromagnetic flowmeter under uniformly turbulent flow conditions and stored as a device constant in the flowmeter,
instantaneous values of the quotient are continuously formed in operation, which are compared with the device constant, and
when a predeterminable threshold is exceeded, an alarm is triggered and/or the volumetric flow rate signal is corrected.
Moreover, the invention provides an electromagnetic flowmeter for a fluid flowing in a line, comprising:
a flow tube connectable into the line conducting the fluid;
a measuring and control circuit;
a coil assembly fed by the measuring and control circuit, said coil assembly producing a magnetic field cutting across the flow tube by means of a first field coil mounted on the flow tube and by means of a second field coil mounted on the flow tube;
at least two measuring electrodes for picking off potentials which are induced in the fluid flowing through the flow tube and traversed by the magnetic field; and
means connected at least intermittently to the measuring electrodes for producing at least one measurement signal derived from the potentials induced in the fluid,
with the first field coil being traversed at least intermittently by a first excitation current, and the second field coil being traversed at least intermittently by a second excitation current,
the two excitation currents being adjusted by means of the measuring and control circuit in such a manner that at least intermittently, a first partial magnetic field, produced by means of the first field coil, has an average strength which is different from an average strength of a second partial magnetic field, produced simultaneously by means of the second field coil.
In a first preferred embodiment of the method of the invention, the strength of at least one of the excitation currents is varied in such a way that the average strengths of the partial magnetic fields are temporarily essentially equal.
In a second preferred embodiment of the method of the invention, the measurement signal is repeatedly sampled to obtain a sequence of discrete sample values which corresponds to a waveform of the induced voltage.
In a third preferred embodiment of the method of the invention, the sampling sequence is stored section by section in a storage means of the measuring and control circuit.
In a fourth preferred embodiment of the method of the invention, a first voltage difference is determined between sample values of the sampling sequence which were each sampled at an instant when the average strengths of the partial magnetic fields are equal.
In a fifth preferred embodiment of the method of the invention, a second voltage difference is determined between sample values of the sampling sequence which were each sampled at an instant when the average strengths of the partial magnetic fields are different from each other and the partial magnetic fields are directed opposite to each other.
In a sixth preferred embodiment of the method of the invention, a volumetric flow rate value is determined by means of the two voltage differences.
In a seventh preferred embodiment of the method of the invention, the volumetric flow rate value is derived from the first voltage difference, and the second voltage difference is used to correct flow-profile-induced deviations of the first voltage difference from the actual volumetric flow rate.
In an eighth preferred embodiment of the method of the invention, the second voltage difference is used to trigger an alarm which signals a flow profile resulting in erroneous measurement signals.
In a ninth preferred embodiment of the method of the invention, a quotient of the two voltage differences is formed for determining the volumetric flow rate and/or triggering the alarm.
In a tenth preferred embodiment of the method of the invention, the quotient is compared with a threshold value which represents a predetermined flow profile to be monitored.
A fundamental idea of the invention is to produce in operation, particularly in the area of the measuring electrodes, two temporarily oppositely directed partial magnetic fields of different strengths to obtain at least intermittently a magnetic field which is measurably asymmetric with respect to the longitudinal axis of the flow tube, whereby even slight deviations from the flow profile to which the flowmeter was calibrated, and/or changes in the coil assembly or in the electrodes can be detected. Thus, based on the detection of such deviations from the calibrated standard, the measurement results can be corrected in operation or at least an alarm signaling such deviations can be triggered.
The invention is also predicated on the surprising recognition that in the tube cross section, the partial magnetic fields superposition with direction-dependent weighting, giving the magnetic field.
It is an advantage of the invention that unforeseeable flow-profile instabilities and changes occurring, particularly spontaneously, during measurements can be continuously and reliably detected, indicated, and/or corrected. Particularly with regard to the flowmeters disclosed in the above-mentioned U.S. Pat. No. 5,646,353, in which the induced voltage is taken off not along a diameter, but along a chord lying in the cross section of the flow tube, another advantage of the invention is that virtually without any major changes in the mechanical construction of conventional flowmeters, besides the instantaneous flow profile and/or its change, viscosities of non-Newtonian fluids can be determined.